The present invention relates generally to a method for removing metal cladding that is adhered to at least one surface of the substrate of an airfoil, such as a turbine (fan) blade, by treating the metal cladding with a chemical etchant. The present invention particularly relates to a method for removing titanium cladding that is adhered at or along the leading edge of the substrate of an airfoil, such as a turbine blade, to a non-metallic fiber reinforced resin substrate by treating the titanium cladding with a chemical etchant that comprises hydrofluoric acid.
High bypass turbofan engines are widely used for high performance aircraft that operate at subsonic speeds. Large fans are typically placed at the front of such engines, and serve to produce greater thrust and reduce specific fuel consumption. To minimize engine weight while optimizing performance, it is often preferred to incorporate a maximum number of fan blades having a fixed root blade configuration (i.e., the blade is dovetailed into a hub). Weight reduction can be achieved in high bypass engines with the use of composite fan blades formed from strong but lightweight materials such as aluminum and graphite fiber-reinforced epoxies. Use of these composite materials also allows for ease of manufacture of relatively complex contours in the blade.
Composite blades are susceptible to foreign object damage. Many types of foreign objects can be entrained in the inlet of an aircraft gas turbine engine ranging from large birds, such as seagulls, to hailstones, sand and rain. Smaller objects can erode the blade material and degrade the performance of the compressor. Impact by larger objects can rupture or pierce the blades with portions of an impacted blade being torn loose and causing extensive secondary damage to downstream blades and other engine components.
The vulnerability of these composite blades to foreign object damage is due to two factors. First, the lightweight matrix material employed for the substrate of the blade, such as polymeric resins or aluminum, can be susceptible to erosion. Second, when high strength filaments are used, these materials can be relatively brittle compared to metals and metal alloys, such as titanium. To provide the lightweight benefit of composites with the protection of metal, hybrid turbine fan blades have been made that comprise a composite substrate having an airfoil shape which is covered by a surface metal cladding, such as titanium cladding. Typically only the blade tip edge and the leading and trailing edge portions have such surface metal cladding which is usually bonded or adhered by a suitable adhesive to the composite substrate. Being less brittle, this metal cladding provides protection to the edges of the turbine fan blade, especially with regard to the leading edge of the blade, against foreign object damage and erosion.
Even this metal cladding adhered to the edges of the turbine fan blade can become dented and damaged due to foreign objects, and thus require repair or replacement. The prior method for removing such metal cladding from the leading, trailing and tip edges is by mechanical removal using a tool that can grasp the cladding. The cladding that is grasped by the tool is then pried or peeled away from the composite substrate to which it is adhered. Unfortunately, as the metal cladding is peeled away, it can strip off or otherwise damage the underlying composite substrate. This is particularly the case where the metal cladding is adhered to the composite substrate at or along an edge (e.g., leading edge) having a compound series of curves. In addition, metal claddings that are relatively strong and less malleable can tear (rather than bend) during this peeling operation, making it more difficult to remove the cladding. Tearing of the metal cladding is also more likely to occur when the cladding varies in thickness in the direction from the interior of the exposed blade surface (typically relatively thin) to the perimeter of the edge (typically relatively thick).
Accordingly, it would be desirable to be able to easily remove dented or damaged metal cladding adhered at or along the edge of a turbine fan blade or other airfoil without stripping away or otherwise damaging the underlying composite substrate, even when the edge has a compound series of curve and/or when the metal cladding is relatively strong and less malleable.